JP5952656B2 - Manufacturing method of glass preform for optical fiber - Google Patents

Description

  The present invention relates to a method for manufacturing a glass preform for an optical fiber.

  The optical fiber glass base material is, for example, deposited on the surface of a silica glass rod manufactured by the VAD method and deposited externally to form a silica porous base material for optical fiber, which is sintered. And can be produced by forming into a transparent glass. Further, usually, immediately before the deposition of the quartz glass fine particles, the surface of the quartz glass rod is etched with a flame to remove foreign matters.

  The etching process is usually performed by moving a torch or a burner that generates a flame relative to the rod along the longitudinal direction of the quartz glass rod. And as a flame, a plasma flame or an oxyhydrogen flame is utilized. In particular, unlike an oxyhydrogen flame, a plasma flame does not require the use of hydrogen gas during generation (see Patent Document 1). Therefore, water is not generated during the etching process, and the hydroxyl group (—OH) on the etching surface of the quartz glass rod is not generated. ) Can be suppressed to a low level. When the remaining hydroxyl group is suppressed, the resulting optical fiber has reduced transmission loss (hereinafter sometimes abbreviated as OH loss) due to this, and has more preferable optical characteristics.

  On the other hand, in order to perform sufficient etching so that the amount of etching with the quartz glass rod reaches the target value, the above etching treatment is performed not only once but two or more times (multiple times) regardless of the type of flame. ) It may be necessary to repeat.

Japanese Patent Publication No. 4-79981

  However, when the above etching process is repeated a plurality of times, the etching amount varies in the longitudinal direction of the quartz glass rod, and the quartz glass rod after the etching process may vary greatly in the outer diameter in the longitudinal direction. There was a problem. When the silica glass fine particles are deposited on the etched surface of the quartz glass rod having the outer diameter fluctuating in this way, the amount of the fine particles deposited decreases at a portion having a small outer diameter, and at a portion having a large outer diameter. It tends to increase. As a result, in the finally obtained optical fiber glass preform, the ratio of the outer diameter of the core to the outer diameter of the cladding (the outer diameter of the core / the outer diameter of the cladding) varies greatly in the longitudinal direction. Therefore, the ratio of the outer diameter of the core and the outer diameter of the clad in the optical fiber obtained from the optical fiber varies greatly in the longitudinal direction. Such an optical fiber has a large amount of fluctuation of the cutoff wavelength, and has undesirable optical characteristics.

  The present invention has been made in view of the above circumstances, and is a method for manufacturing a glass preform for an optical fiber, which includes a step of etching a quartz glass rod while suppressing fluctuations in the etching amount of the surface in the longitudinal direction. It is an object to provide a method.

To solve the above problem,
In the present invention, after the surface of the quartz glass rod is etched with a flame, quartz glass fine particles are deposited on the etched surface to form a quartz porous preform for optical fiber, and the quartz porous preform for optical fiber is transparent. A method of manufacturing a glass preform for an optical fiber, comprising a step of vitrifying, wherein the etching treatment is performed on a quartz glass rod along a longitudinal direction of a torch that generates the flame only in one direction. A plurality of times of relative movement, and the relative movement of the torch with respect to the quartz glass rod is started from one end of the quartz glass rod and directed toward the other end. I will provide a.
According to such a manufacturing method, the quartz glass rod has a suppressed variation in surface etching amount (outer diameter) in the longitudinal direction, and the obtained optical fiber glass preform has an outer diameter of the core in the longitudinal direction. And the outer diameter of the cladding (core outer diameter / cladding outer diameter) variation is suppressed. And the fluctuation value of a cut-off wavelength is suppressed in the optical fiber obtained from this preform | base_material.
In the method for manufacturing a glass preform for an optical fiber according to the present invention, the etching treatment is performed by moving the torch for generating the flame a plurality of times in only one direction along the longitudinal direction with respect to the quartz glass rod. It is preferable that the torch is moved relatively from one end to the other end of the quartz glass rod by one relative movement.

In the method for manufacturing a glass preform for an optical fiber according to the present invention, it is preferable to calculate an etching amount by measuring an outer diameter of the quartz glass rod before and after the etching process.
By calculating the etching amount, the value can be compared with the target value to determine each time whether or not the etching is performed to the target level.

In the method for producing a glass preform for an optical fiber according to the present invention, after calculating the etching amount, the value is compared with a target value, and the torch is attached to the quartz glass rod along the longitudinal direction. It is preferable to determine whether or not an etching process is added by performing relative movement only in the direction.
By determining whether or not the etching process is added and adding the etching process as necessary, the quartz glass rod can be reliably etched to the target level.

In the method for producing a glass preform for an optical fiber of the present invention, the flame is preferably a plasma flame.
By using the plasma flame, the generation of water during the etching process is suppressed, the remaining hydroxyl groups on the etching surface of the quartz glass rod are suppressed, and an optical fiber with further reduced OH loss can be obtained.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the glass preform | base_material for optical fibers which has the process of suppressing the fluctuation | variation of the etching amount of the surface in the longitudinal direction, and the quartz glass rod is provided.

It is a schematic diagram for demonstrating the manufacturing method of the glass preform for optical fibers which concerns on this invention. It is a schematic sectional drawing which illustrates the glass base material for optical fibers obtained by this invention, and the relationship of the refractive index of each layer which comprises this base material. It is a flow figure for explaining one embodiment of a manufacturing method of a glass preform for optical fibers concerning the present invention. It is a graph which shows the calculation result of the outer diameter difference of the quartz glass rod which arose in the example 1 and 2 of the etching process twice.

The method for producing a glass preform for an optical fiber according to the present invention, after etching the surface of the quartz glass rod with a flame, deposits quartz glass fine particles on the etched surface to obtain a quartz porous preform for an optical fiber, A method for producing a glass preform for optical fiber, comprising the step of converting the quartz porous preform for optical fiber into transparent glass, wherein the etching treatment is performed along the longitudinal direction of the quartz glass rod. The torch that generates the flame is relatively moved multiple times in only one direction.
According to such a manufacturing method, during the etching process, the quartz glass rod has a more uniform surface temperature during the etching process in the longitudinal direction. Therefore, the etching amount of the surface (etching from the surface of the quartz glass rod) Variation in distance) is suppressed. As a result, the obtained glass preform for optical fiber has a suppressed variation in the ratio of the outer diameter of the core to the outer diameter of the cladding (the outer diameter of the core / the outer diameter of the cladding) in the longitudinal direction. Become. Therefore, the optical fiber obtained from such a glass preform for optical fiber has excellent optical characteristics in which the variation value of the cutoff wavelength is suppressed.
Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic view for explaining a method for producing an optical fiber glass preform according to the present invention.

  In the present invention, first, the surface of the quartz glass rod is etched with a flame. Etching treatment removes various foreign substances, such as water, existing in advance on the surface of the quartz glass rod.

The quartz glass rod may ultimately constitute the core and part of the clad in the optical fiber glass preform and optical fiber, or may constitute the core, part of the clad, and the trench layer, Only the core may be configured, and can be arbitrarily selected according to the purpose.
The size of the quartz glass rod is not particularly limited.
The quartz glass rod can be produced by a known method such as the VAD method.

  Examples of the flame for the etching treatment include an oxyhydrogen flame and a plasma flame. Of these, unlike oxyhydrogen flames, plasma flames do not require the use of hydrogen gas during production, so water is not generated during the etching process, and the remaining hydroxyl groups on the etched surface of the quartz glass rod are suppressed. Is preferable.

The method of generating the plasma flame may be selected as appropriate in consideration of handling, safety, type of heat source, etc., but there is a method of generating a plasma flame by applying a voltage to the gas serving as the plasma source and sparking it. preferable.
Argon (Ar) gas can be illustrated as a gas used as a plasma source.
The voltage applied during the generation of the plasma flame is preferably a high-frequency voltage, and the frequency of the voltage is preferably 2 MHz to 2.45 GHz.

The etching gas added to the plasma flame is preferably a fluorine-containing gas.
The fluorine-containing gas may be a gas containing a fluorine atom in its chemical structure. Preferred from the viewpoint of etching ability and cost, sulfur hexafluoride (SF ₆ ), hexafluoroethane (C ₂ F) ₆ ), silicon tetrafluoride (SiF ₄ ), and tetrafluoromethane (CF ₄ ).
A fluorine-containing gas may be used individually by 1 type, and may use 2 or more types together.

Depending on the type of etching gas, oxygen (O ₂ ) gas may be further added to the plasma flame as a combustion-supporting gas to promote decomposition of the etching gas. For example, when C ₂ F ₆ gas is used as the etching gas, it is preferable to use oxygen gas in combination.

  In the present invention, it is preferable to perform etching treatment by adding a fluorine-containing gas to a plasma flame generated using argon gas, and etching is performed by further adding oxygen gas to the plasma flame as necessary. More preferably, the treatment is performed.

  The etching process is performed by moving the torch (flame generating means) that generates the flame relative to the quartz glass rod along the longitudinal direction thereof. Here, “relatively moving the torch relative to the quartz glass rod” means (I) fixing the quartz glass rod to move the torch, (II) fixing the torch and moving the quartz glass rod, III) This means that the quartz glass rod and the torch are moved together (except when the absolute value of the moving speed and the moving direction are both the same).

Both the quartz glass rod and the torch are moved along the longitudinal direction (center axis direction) of the quartz glass rod. There are two directions of movement at this time, but any direction may be used. For example, in the case of (III), the quartz glass rod and the torch may be moved in the same direction or in the opposite direction.
In the present invention, the etching process is performed by moving the torch relative to the quartz glass rod a plurality of times in only one direction along the longitudinal direction thereof. In this way, during the etching process, the quartz glass rod has a more uniform surface temperature during the etching process in the longitudinal direction, and therefore the etching amount becomes more uniform and the surface etching amount varies. Is suppressed.

FIG. 1A illustrates one embodiment of the above etching process.
The quartz glass rod 10 has a dummy base material 9 coaxially connected to both ends, and these dummy base materials 9 are held by a pair of rotating chucks 8 and 8. That is, the quartz glass rod 10 is rotatable around the axis as indicated by the arrow A and is installed in the reaction vessel 7.
One side of the reaction vessel 7 is connected to an exhaust duct (not shown), and the inside of the exhaust duct is usually at a negative pressure of about 20 to 300 Pa. By supplying clean air from the outside of the reaction vessel 7, the reaction vessel 7 The inside of the booth (not shown) where 7 is placed is adjusted so as to have a positive pressure of about 3 to 30 Pa.
Here, the quartz glass rod 10 is rotated in the arrow A direction, the quartz glass rod 10 is fixed in the longitudinal direction, and the torch 6 is moved in the arrow B direction (left side) along the longitudinal direction, The case where an etching process is performed is illustrated. Reference numeral 4 denotes a flame. The quartz glass rod 10 may be rotated in the opposite direction to the arrow A direction around the same axis, and the torch 6 may be moved in the arrow C direction (right side) which is the opposite direction to the arrow B direction. Good. The rotation speed of the quartz glass rod 10 is preferably 15 to 40 rpm, for example.

In the present invention, in the case of FIG. 1A, during the etching process, the torch 6 is relatively moved only in the direction of the arrow B without relative movement in the direction of the arrow C, or the torch 6 is relatively moved in the direction of the arrow B. Relative movement is performed a plurality of times only in the direction of arrow C without being moved.
In one relative movement in the direction of arrow B, for example, the torch 6 is relatively moved from one end 10a to the other end 10b of the quartz glass rod 10 in the longitudinal direction of the quartz glass rod 10, and once in the arrow C direction. In the relative movement, for example, relative movement is performed in a desired region in the longitudinal direction of the quartz glass rod 10, such as relative movement from the other end 10 b of the quartz glass rod 10 to one end 10 a in the longitudinal direction of the quartz glass rod 10. And the start position and the end position of the relative movement of the torch 6 may be appropriately selected according to the purpose.

  When the torch 6 is relatively moved not only in one direction but also in two directions (arrow B direction and C direction), the surface of the quartz glass rod 10 in the vicinity of the inverted portion before and after reversing the direction of relative movement is With the inversion of 6, the continuous heating time by the flame 4 becomes longer than other surfaces. For example, when the torch 6 is relatively moved in the direction of the arrow B to reach the vicinity of the other end 10b of the quartz glass rod 10, the direction is reversed, and when the relative movement is performed in the direction of the arrow C, the reversal is performed. The surface of the quartz glass rod 10 in the vicinity of the part (the surface in the vicinity of the other end 10b) has a longer continuous heating time due to the flame 4 than, for example, the central portion in the longitudinal direction of the quartz glass rod 10 or the surface in the vicinity thereof. Then, the surface of the quartz glass rod 10 in the vicinity of the reversal site is etched in a preheated state compared to the surface of other regions such as the central portion, so that the etching is accelerated and the etching amount increases. . As a result, the other end 10b of the quartz glass rod 10, that is, the vicinity of the inverted portion of the torch 6, has a smaller outer diameter than other regions such as the central portion. Contrary to the above, the torch 6 is relatively moved in the direction of arrow C to reach the vicinity of the one end 10a of the quartz glass rod 10, and then the direction is reversed to be relatively moved in the direction of arrow B. The same applies to the case. In this case, the surface of the quartz glass rod 10 in the vicinity of the inversion site (the surface in the vicinity of the one end 10a) has a longer continuous heating time due to the flame 4 than the surface of other regions such as the central portion, and the quartz One end 10a of the glass rod 10, that is, the vicinity of the inverted portion of the torch 6, has a smaller outer diameter than other regions such as the central portion.

When the length of the quartz glass rod 10 in the longitudinal direction is short, for example, when the length of the etching target region is about 500 mm, the influence of the continuous heating time (preheating degree) appears strongly, and the quartz glass rod The outer diameter of 10 has a large variation in the entire area in the longitudinal direction.
On the other hand, when the length of the quartz glass rod 10 in the longitudinal direction is long, for example, when the length of the etching target region is about 1500 mm, the time required for the torch 6 to complete one relative movement becomes long. The central portion in the longitudinal direction of the quartz glass rod 10 and its peripheral region require a relatively long time until the etching process by the next relative movement of the torch 6, and the temperature is likely to be stabilized by cooling. As a result, in this region of the quartz glass rod 10, the etching amount by one etching process is stabilized, and the fluctuation of the outer diameter is suppressed. However, the area of the quartz glass rod 10 in the vicinity of the reversal part of the torch 6 remains the same as that of the central part and the peripheral area due to the influence of the continuous heating time. The fluctuation of becomes large.
As described above, when the torch 6 is relatively moved in two directions, the quartz glass rod 10 after the etching process varies in the outer diameter in the longitudinal direction regardless of the length of the quartz glass rod 10 in the longitudinal direction. Becomes larger.

  On the other hand, in the present invention, the quartz glass rod 10 has its longitudinal portion because the inversion portion as described above does not occur by moving the torch 6 only in the direction of arrow B or in the direction of arrow C a plurality of times. In the direction, the surface temperature when the etching process is performed becomes more uniform, and the etching amount becomes uniform. As a result, the quartz glass rod 10 becomes uniform with fluctuations in the outer diameter being suppressed in the longitudinal direction.

During the etching process from the end of one relative movement to the start of the next relative movement, the torch is heated by the quartz glass rod during the movement from the end position of the etching process to the start position. Adjust appropriately so that it does not receive. Here, “effect of heating” means “fluctuation of etching amount in the longitudinal direction of the quartz glass rod”.
For example, it is preferable to move the torch 6 while extinguishing the flame from the end position of the etching process to the start position between the etching processes. When the flame is not extinguished, the torch is moved from the end position of the etching process to the start position so that the flame is sufficiently separated from the quartz glass rod between the etching processes.

  The absolute value of the relative movement speed of the torch may be constant or fluctuated between the start and end of the etching process, but is preferably constant. By making it constant, the effect of suppressing fluctuations in the etching amount in the longitudinal direction of the quartz glass rod is enhanced.

  The absolute value of the relative movement speed of the torch in the etching process can be arbitrarily set so as to have a desired etching amount without being affected by other processes.

In the present invention, it is preferable to measure the outer diameter of the quartz glass rod before and after the etching process. And it is preferable to calculate the etching amount accompanying an etching process from the outer diameter of the quartz glass rod before and behind an etching process. By calculating the etching amount, the value can be compared with the target value to determine whether or not the etching is performed to the target level, that is, whether or not the etching process is added each time, and the etching amount is the target value. If it has not reached, an additional etching process may be performed. Even if the etching amount by one etching process is known in advance, the etching amount by one etching process is the expected value depending on the size of the quartz glass rod used, the viscosity of the quartz glass, the presence or absence of equipment trouble, etc. In this case, it may be controlled to perform an additional etching process. By doing in this way, an etching process can be reliably performed to the target grade with respect to a quartz glass rod.
The direction of the relative movement of the torch in the additional etching process may be the same as or different from the etching process performed before this, but is preferably the same.
The outer diameter of the quartz glass rod can be measured by a known method using, for example, a laser outer diameter measuring machine.

  After the etching process, it is preferable to pre-heat the etched surface of the quartz glass rod separately from this (etching process). By performing the pre-heat treatment in this manner, the quartz glass fine particles, which will be described later, are deposited on the etched surface of the quartz glass rod in a heated state, so that the adhesion to the treated surface is improved and the quartz glass rod is improved. The occurrence of peeling and deviation between the outer layer and the external layer is suppressed. In addition, when the quartz glass fine particles are deposited a plurality of times, the quartz glass fine particles deposited after the second time are deposited on the surface (quartz glass) by using a flame having a high temperature for the deposition of the quartz glass fine particles. Since the temperature of the fine particle deposition layer surface is sufficiently high, the degree of adhesion with the surface is high.

The pre-heat treatment is preferably performed using a plasma flame. Also in the pre-heat treatment, the use of the plasma flame suppresses the remaining hydroxyl groups on the etched surface of the quartz glass rod.
Furthermore, since the plasma flame has a small heat capacity and does not contain light in the infrared wavelength region, it generates much less radiant heat than the oxyhydrogen flame. Therefore, even if the quartz glass rod is heated with a plasma flame, the surface does not become hot (preheated) for a long time at a high temperature over a wide range, so water is bonded to the etched surface (reaction). ) Cool well before doing. Usually, the higher the temperature of the etched surface, the easier it is for water to bond (react) with the etched surface. Combined with these effects, the residual heat of the hydroxyl group on the etched surface of the quartz glass rod is highly suppressed by performing the pre-heat treatment using the plasma flame.

  The pre-heat treatment may be performed only once for the target quartz glass fine particle deposition site (etching site), or may be repeated twice or more (multiple times). However, normally, sufficient effects can be obtained by performing only once.

The flame at the time of the pre-heat treatment may be generated by the same method as that at the time of the etching process. For example, the plasma flame is preferably generated using argon gas, nitrogen (N ₂ ) gas, or oxygen gas. By using such a plasma flame, the pre-heat treatment can be performed more effectively.

  The pre-heat treatment is preferably performed by moving the torch (flame generating means) relative to the quartz glass rod (quartz glass rod with an etched surface) along the longitudinal direction. Except that the types are different and the direction of relative movement of the torch is not limited to only one direction, it can be performed by the same method as in the etching process described above.

  There are two directions of movement of the torch during the pre-heat treatment along the longitudinal direction (center axis direction) of the quartz glass rod, but either direction may be used, and the direction of movement of the torch during the etching process may be the same. And may be different.

  The absolute value of the relative movement speed of the plasma torch from the start to the end of the preheat treatment may be constant or may be varied, but is preferably constant. By making it constant, in the glass preform for optical fiber described later, the effect of suppressing peeling and deviation between the quartz glass rod and the external layer becomes higher.

  The pre-heat treatment may be performed after the etching process is completed, or may be performed in parallel with the etching process by performing the etching process from the part where the etching process has been completed before the etching process is completed.

After the etching process, quartz glass fine particles are deposited on the etched surface of the quartz glass rod to obtain a quartz porous preform for optical fiber. When pre-heat treatment is performed, quartz glass fine particles may be deposited on the etched surface of the quartz glass rod that has been heated by the pre-heat treatment.
The deposition of the quartz glass fine particles can be performed by a known method.

  The quartz glass fine particles may be deposited only once, or may be repeated two or more times (multiple times). Usually, a sufficient amount of quartz glass fine particles is deposited by repeating plural times. It is preferable to make it. The number of repetitions at this time may be appropriately adjusted according to the size of the target glass preform for optical fiber, and may be, for example, about several hundred times or less, such as several hundred times.

The quartz glass fine particles to be deposited are preferably produced in an oxyhydrogen flame using a quartz glass raw material gas, hydrogen (H ₂ ) gas, oxygen gas and inert gas.
Examples of the quartz glass source gas include silicon tetrachloride (SiCl ₄ ) gas and germanium tetrachloride (GeC 1 ₄ ) gas.
Examples of the inert gas include argon gas and nitrogen gas.

  The deposition temperature (deposition temperature) of the quartz glass fine particles is preferably 500 to 1100 ° C., and more preferably 650 to 1050 ° C.

  The quartz glass fine particles are preferably deposited by moving a quartz glass burner for producing the quartz glass fine particles relative to the quartz glass rod (etched quartz glass rod) along the longitudinal direction thereof. Here, “relatively moving the quartz glass burner relative to the quartz glass rod” is the same as in the case of the torch described above, and (i) the quartz glass rod is fixed and the quartz glass burner is moved (ii) ) The quartz glass burner is fixed and the quartz glass rod is moved. (Iii) The quartz glass rod and the quartz glass burner are moved together (except when the absolute value of the moving speed and the moving direction are both the same). Means either.

Both the quartz glass rod and the quartz glass burner are moved along the longitudinal direction (center axis direction) of the quartz glass rod. There are two directions of movement at this time, but any direction may be used. For example, in the case of (iii), the quartz glass rod and the quartz glass burner may be moved in the same direction or in the opposite direction.
When the quartz glass fine particles are repeatedly deposited a plurality of times, the moving direction may be the same direction at all times, or may be different at all times (two times for each direction). Or may be in different directions only for some times.

  The absolute value of the speed of the relative movement of the quartz glass burner may be constant or fluctuated during the period from the start to the end of the deposition of the quartz glass fine particles, but is preferably constant. By making it constant, the thickness of the quartz glass fine particle deposition layer (soot deposition layer) becomes more uniform in the longitudinal direction of the quartz glass rod.

  When pre-heat treatment is performed, the quartz glass fine particles may be deposited after completion of the pre-heat treatment, or before the pre-heat treatment is completed, by performing the pre-heat treatment at the pre-heated portion. You may perform in parallel with heat processing. And in the glass base material for optical fibers, since the effect which suppresses peeling and a shift | offset | difference between a quartz glass rod and an external layer becomes higher, it is preferable to carry out in parallel with pre-heat treatment. By doing in this way, quartz glass microparticles can be deposited on the etched surface where the degree of heat release (temperature decrease) after pre-heat treatment is low.

FIG. 1B illustrates a preferred embodiment of the deposition of quartz glass fine particles when pre-heat treatment is performed.
Here, the quartz glass rod 10 is fixed in the longitudinal direction, and the pre-heat treatment and the torch 6 and the quartz glass burner 5 are moved in the same direction, that is, the arrow B direction (left side) along the longitudinal direction. The case where quartz glass fine particles are deposited is illustrated. At this time, the quartz glass rod 10 is rotated in the direction of arrow A. The quartz glass burner 5 is moved so as to follow the torch 6. Although no etching gas is added to the flame 4, the torch 6 may perform not only the pre-heat treatment here but also the etching treatment.
The pressure in the exhaust duct and in the booth where the reaction vessel 7 is placed are the same as in the case of FIG.
When the quartz glass fine particles are deposited a plurality of times, it can be deposited by moving only the quartz glass burner 5 in the second and subsequent times.
Further, the quartz glass rod 10 may be rotated in the opposite direction with respect to the arrow A direction around the same axis, and the torch 6 and the quartz glass burner 5 are moved in the opposite direction (right side) with respect to the arrow B direction. May be. The rotation speed of the quartz glass rod 10 is the same as in the case of FIG.
Here, an example is shown in which two quartz glass burners 5 are arranged in series along the longitudinal direction of the quartz glass rod 10, but the number and arrangement of the quartz glass burners 5 are not limited thereto. Can be adjusted as appropriate.

After the quartz glass fine particles are deposited on the etched surface of the quartz glass rod, the obtained quartz porous preform for optical fiber is made into transparent glass.
Transparent vitrification may be performed by a known method, and can be performed by dehydrating and sintering a quartz porous base material for optical fibers. However, the base material has a sufficiently reduced amount of hydroxyl groups. In some cases, the dehydration process may be omitted.

The dehydration treatment is preferably performed in the presence of a chlorine-containing gas.
The sintering treatment is preferably performed in the presence of an inert gas such as helium gas.
In addition, by performing dehydration and sintering treatment in the presence of a fluorine-containing gas, fluorine can be added to reduce the refractive index of the layer (external layer) that has been made into a transparent glass. As the fluorine-containing gas at this time, the same fluorine-containing gas used as the etching gas in the etching process can be exemplified, and such fluorine-containing gas may be used alone or in combination of two or more. Also good.

After forming into a transparent glass, the resulting product may be used as a glass base material for an optical fiber, or an additional external layer may be formed on the outside of the formed transparent vitrified layer (external layer). It is good also as a glass base material.
The external layer to be additionally formed may be formed by a known method, or the one obtained by the above transparent vitrification is newly used as a quartz glass rod, and the above-described production method of the present invention is applied. It may be formed.
The number and type of external layers to be additionally formed can be arbitrarily set according to the structure of the target glass preform for optical fiber.

  In the present invention, since an etched quartz glass rod in which the fluctuation of the outer diameter in the longitudinal direction is suppressed is used, the formed transparent vitrified layer has a thickness fluctuation that is suppressed in the longitudinal direction. It becomes.

FIG. 2 is a schematic cross-sectional view illustrating an optical fiber glass preform obtained according to the present invention and the relationship between the refractive indexes of the layers constituting the preform. 2A shows the glass preform 1 for an optical fiber in which the core 11, the inner cladding 12 and the outer cladding 13 are provided in this order. FIG. 2B shows the core 11, the inner cladding 12, and the trench layer 14. And the glass preform | base_material 2 for optical fibers in which the outer side cladding 13 was provided in this order is shown.
In FIG. 2, D indicates the outer diameter of the outer cladding, d ₁ indicates the outer diameter of the inner cladding, and d ₂ indicates the outer diameter of the trench layer.
The optical fiber glass preform shown here is merely an example of what can be obtained by the present invention, and the optical fiber glass preform obtained by the present invention is not limited to this.

The glass preform 1 for an optical fiber can be manufactured, for example, by using a quartz glass rod that constitutes the core 11 and the inner cladding 12 and applying the present invention when forming the outer cladding 13.
Moreover, the glass preform 2 for optical fiber can be manufactured by applying the present invention when forming the trench layer 14 using, for example, a quartz glass rod that constitutes the core 11 and the inner cladding 12.

  FIG. 3 illustrates a flow diagram for explaining an embodiment of the method for producing an optical fiber glass preform according to the present invention. Of the steps shown here, the step surrounded by a dotted frame can be omitted depending on the situation.

According to the present invention, in the manufacturing process of the optical fiber glass preform, the quartz glass rod that has been subjected to the etching treatment is suppressed from changing in the outer diameter in the longitudinal direction. For example, when a quartz glass rod having an outer diameter of 20 to 45 mm and a length of 300 to 2000 mm is subjected to an etching process, the outer diameter difference before and after the etching process (twice the etching amount) in the longitudinal direction of the quartz glass rod. Value) can be suppressed to preferably 0.06 mm or less, more preferably 0.05 mm or less.
And the fluctuation | variation of the thickness in the longitudinal direction of the transparent vitrification layer attached externally is suppressed by using the quartz glass rod by which such an etching process was carried out. In other words, in the longitudinal direction, fluctuations in the ratio between the outer diameter of the core and the outer diameter of the cladding (the outer diameter of the core / the outer diameter of the cladding) are suppressed. Reflecting this characteristic, the optical fiber obtained from the glass preform for optical fiber has excellent optical characteristics in which the variation value of the cutoff wavelength is suppressed in the longitudinal direction. For example, an optical fiber having a cutoff wavelength standard deviation (σ) of preferably 21.5 nm or less, more preferably 20.0 nm or less is obtained.

  Hereinafter, the present invention will be described in more detail with reference to specific examples. However, the present invention is not limited to the following examples.

<Etching treatment of quartz glass rod surface and calculation of etching amount (outer diameter difference)>
[Experiment 1]
The surface of the quartz glass rod was etched by the method described with reference to FIG. Specifically, it is as follows.
A dummy base material is coaxially connected to both ends of a quartz glass rod manufactured by the VAD method, and this is gripped by a pair of rotating chucks in a reaction vessel, and then the quartz glass rod is rotated around the axis at a rotational speed of 25 rpm. It was. A quartz glass rod having a length of about 500 mm was used. In addition, clean air was supplied to the reaction vessel from the outside, and the pressure in the booth where the reaction vessel was placed was adjusted to fall within the range of 3 to 30 Pa.
Next, the quartz glass rod was fixed in the longitudinal direction, and the surface of the quartz glass rod was etched with a plasma flame by moving the plasma torch at a speed of 15 mm / min along the longitudinal direction. At this time, argon gas was supplied to the plasma torch at a flow rate of 30 SLM, and a high frequency plasma flame was generated by applying a voltage under the conditions of 13.56 MHz and 6 KW using a high frequency power source. Then, an etching process was performed by adding SF ₆ gas to the generated plasma flame at a flow rate of 2.5 SLM. The etching treatment was performed twice by moving the plasma torch twice in only one direction from one end to the other end of the glass rod. In addition, before and after the etching process, the outer diameter of the quartz glass rod was measured in the entire longitudinal direction using a laser outer diameter measuring machine (LS-3000 series manufactured by Keyence Corporation).
Then, the difference between the measured value of the outer diameter of the quartz glass rod before the first etching process and the measured value of the outer diameter of the quartz glass rod after the second etching process (quartz glass produced by the second etching process). The outer diameter difference of the rod was calculated over the entire length of the quartz glass rod. The result at this time is shown in FIG. The etching amount on the surface of the quartz glass rod by the etching process twice is a value of 1/2 of the difference in outer diameter.

[Experiment 2]
After moving the plasma torch once from one end to the other end of the quartz glass rod, immediately reverse the direction and move once in the opposite direction toward one end, moving it once in two directions. Except for this, the etching process was performed in the same manner as in Experimental Example 1, and the difference in the outer diameter of the quartz glass rod produced by the two etching processes was calculated. The result at this time is shown in FIG.

As shown in FIG. 4, in Experimental Example 1, the quartz glass rod is moved from one end (one etching processing start position, a position where the horizontal axis in the drawing is near 0 mm) to the other end (one etching processing end position, FIG. The variation of the outer diameter difference was suppressed over the entire area in the longitudinal direction, with the horizontal axis in the vicinity of the region around 500 mm.
On the other hand, in Experimental Example 2, the difference in outer diameter of the quartz glass rod fluctuated over the entire region in the longitudinal direction from one end to the other end. Specifically, the difference in outer diameter of the quartz glass rod generally increased from one end to the other end. This is because the other end is located in the vicinity of the reversal part of the plasma torch, and the closer to the other end, the longer the surface of the quartz glass rod is continuously heated by the flame, and the second etching process is preheated. This was because etching was promoted. In Experimental Example 2, the difference between the outer diameters in the region where the horizontal axis is in the vicinity of 0 mm and 500 mm greatly fluctuates and noise is generated. This is because the outer diameter of the dummy base material was measured.

[Example 1]
<Manufacture of glass preforms for optical fibers>
By the method described with reference to FIG. 1, an optical fiber glass preform having the structure shown in FIG. 2B was manufactured under the conditions shown in Table 1. Specifically, it is as follows.
A dummy base material is coaxially connected to both ends of a quartz glass rod manufactured by the VAD method, and this is gripped by a pair of rotating chucks in a reaction vessel, and then the quartz glass rod is rotated around the axis at a rotational speed of 25 rpm. It was. As the quartz glass rod, a rod having a length of 1500 mm and constituting a core and an inner cladding was used. In addition, clean air was supplied to the reaction vessel from the outside, and the pressure in the booth where the reaction vessel was placed was adjusted to fall within the range of 3 to 30 Pa.

Next, the quartz glass rod was fixed in the longitudinal direction, and the surface of the quartz glass rod was etched with a plasma flame by moving the plasma torch along the longitudinal direction. At this time, argon gas was supplied to the plasma torch at a flow rate of 30 SLM, and a high frequency plasma flame was generated by applying a voltage under the conditions of 13.56 MHz and 6 KW using a high frequency power source. Then, an etching process was performed by adding SF ₆ gas to the generated plasma flame at a flow rate of 2.5 SLM. The etching treatment was performed twice by moving the plasma torch twice in only one direction from one end to the other end of the glass rod. In addition, before and after the etching process, the outer diameter of the quartz glass rod was measured in the entire longitudinal direction using a laser outer diameter measuring machine (LS-3000 series manufactured by Keyence Corporation).
Then, the difference between the measured value of the outer diameter of the quartz glass rod before the first etching process and the measured value of the outer diameter of the quartz glass rod after the second etching process (quartz glass produced by the second etching process). The outer diameter difference of the rod was calculated over the entire length of the quartz glass rod.

Next, after the etching process is completed, the quartz glass rod is fixed in the longitudinal direction, and the plasma torch and the quartz glass burner are moved in the same direction along the longitudinal direction (the quartz glass is chased to follow the plasma torch). By moving the burner), pre-heat treatment and quartz glass fine particle deposition were performed once in parallel. The pre-heat treatment was performed using a high-frequency plasma flame generated in the same manner as in the etching process except that SF ₆ gas was not added. The quartz glass fine particles were deposited by being produced in an oxyhydrogen flame using quartz glass raw material gas, hydrogen gas and oxygen gas, and further using argon gas and nitrogen gas as inert gases. The pre-heat treatment and the deposition of the quartz glass fine particles were performed by moving the plasma torch and the quartz glass burner once in one direction from one end of the glass rod to the other end.
Next, after retracting the plasma torch to a position that does not hinder the movement of the quartz glass burner, the quartz glass fine particles were deposited several times by repeatedly moving only the quartz glass burner in the same direction as the first time. .
Through the steps so far, a quartz porous preform for an optical fiber was produced.

Next, the obtained silica porous preform for optical fiber is put into a sintering furnace, dehydrated in a chlorine-containing gas atmosphere, and then subjected to sintering and fluorine addition simultaneously in a mixed gas of SiF ₄ and helium. It was.
Through the steps so far, the core-trench layer portion of the optical fiber glass preform having the structure shown in FIG.

Next, an outer cladding was formed by a conventional external method including dehydration treatment in a chlorine-containing gas atmosphere.
The glass base material for optical fibers having the structure shown in FIG.

<Manufacture of optical fiber>
The obtained glass preform for optical fiber was made into a strand by a conventional method to produce an optical fiber having an outer clad outer diameter of 125 μm. And about the obtained optical fiber, the cutoff wavelength and OH loss were measured, and the standard deviation ((sigma)) of the cutoff wavelength was calculated | required. The results are shown in Table 1.

[Example 2]
As shown in Table 1, a glass preform for an optical fiber was manufactured in the same manner as in Example 1 except that a quartz glass rod having a length of 500 mm was used, and an optical fiber was further manufactured. Manufactured.
Table 1 shows the standard deviation (σ) of the cutoff wavelength and the OH loss of the obtained optical fiber.

[Example 3]
As shown in Table 1, a quartz glass rod having an outer diameter of 40 mm was used, and the etching rate was the same as in Example 1 except that the oxyhydrogen flame was used with a torch moving speed of 55 mm / min. In addition, an optical fiber glass base material was manufactured, and an optical fiber was further manufactured.
Table 1 shows the standard deviation (σ) of the cutoff wavelength and the OH loss of the obtained optical fiber.

[Comparative Example 1]
As shown in Table 2, during the etching process, after moving the plasma torch once from one end of the quartz glass rod to the other end, the direction is immediately reversed and moved once in the opposite direction toward one end. Thus, a glass preform for optical fiber was manufactured in the same manner as in Example 1 except that it was moved once in two directions, and an optical fiber was further manufactured.
Table 2 shows the standard deviation (σ) of the cutoff wavelength and OH loss of the obtained optical fiber.

[Comparative Example 2]
As shown in Table 2, during the etching process, the moving speed of the plasma torch was 80 mm / min, and after moving the plasma torch once from one end of the quartz glass rod to the other end, the direction was immediately reversed, Example 1 except that it was moved once in one direction in the opposite direction, moved once in two directions, and this operation was repeated once to perform the etching process a total of four times. Similarly, an optical fiber glass preform was manufactured, and an optical fiber was manufactured.
Table 2 shows the standard deviation (σ) of the cutoff wavelength and OH loss of the obtained optical fiber.

[Comparative Example 3]
As shown in Table 1, a quartz glass rod having an outer diameter of 40 mm is used, and during the etching process, an oxyhydrogen flame is used with a torch moving speed of 55 mm / min, and the torch is moved from one end of the quartz glass rod. After moving once toward the end, immediately reverse the direction, and move once in the opposite direction toward one end, so that it is moved once in two directions, the same as in Example 1. In addition, an optical fiber glass base material was manufactured, and an optical fiber was further manufactured.
Table 2 shows the standard deviation (σ) of the cutoff wavelength and OH loss of the obtained optical fiber.

As shown in Tables 1 and 2, in Examples 1 to 3, the fluctuation value of the outer diameter difference of the quartz glass rod is 0.02 to 0.03 mm (the fluctuation value of the etching amount is 0.01 to 0.015 mm). As a result, the obtained optical fiber had excellent optical characteristics with the standard deviation (σ) of the cutoff wavelength, that is, the fluctuation value suppressed.
On the other hand, in Comparative Examples 1 to 3, the fluctuation value of the outer diameter difference of the quartz glass rod is as large as 0.08 to 0.10 mm (the fluctuation value of the etching amount is 0.04 to 0.05 mm). The obtained optical fiber had a large fluctuation value of the cutoff wavelength.
The OH loss of the optical fiber was the same in Examples 1 to 3 and Comparative Examples 1 to 3.
As described above, it was confirmed that the present invention has a remarkably excellent effect. This is more apparent from a comparison between Example 1 and Comparative Examples 1 and 2 and a comparison between Example 3 and Comparative Example 3.

  The present invention can be used for manufacturing optical fibers.

  DESCRIPTION OF SYMBOLS 1, 2 ... Glass base material for optical fibers, 10 ... Quartz glass rod, 11 ... Core, 12 ... Inner clad, 13 ... Outer clad, 14 ... Trench layer, 4. .. Flame, 5 ... Quartz glass burner, 6 ... Torch, 7 ... Reaction vessel, 8 ... Rotary chuck, 9 ... Dummy base material

Claims (5)

A step of etching the surface of the quartz glass rod with a flame and then depositing quartz glass fine particles on the etched surface to form a quartz porous preform for optical fiber, and converting the quartz porous preform for optical fiber to transparent glass A method for producing a glass preform for an optical fiber, comprising:
The etching process moves the torch for generating the flame relative to the quartz glass rod a plurality of times in only one direction along the longitudinal direction, and the relative movement of the torch with respect to the quartz glass rod is A method for producing a glass preform for an optical fiber, which is performed from one end of a quartz glass rod toward the other end . In the etching process, the quartz glass rod is moved relative to the quartz glass rod a plurality of times in only one direction along the longitudinal direction, and one end of the quartz glass rod is moved by one relative movement. The method for producing a glass preform for optical fiber according to claim 1, wherein the torch is moved relative to the other end. The method for producing a glass preform for an optical fiber according to claim 1 or 2 , wherein an outer diameter of the quartz glass rod is measured before and after the etching process to calculate an etching amount. After calculating the etching amount, the value is compared with a target value, and the torch is moved relative to the quartz glass rod in only one direction along the longitudinal direction. Presence or absence is judged, The manufacturing method of the glass base material for optical fibers of Claim 3 characterized by the above-mentioned. The said flame is a plasma flame, The manufacturing method of the glass base material for optical fibers as described in any one of Claims 1-4 characterized by the above-mentioned.

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